I started another thread about KSG mods a while ago there is some good info in there, just search KSG. It would be my suggestion to use a Magpul XTM handstop kit for the pump, that's what I did works great. IMO, far better than a vert grip or angled foregrip, I tried most of what's out there(Tango down stubby, Magpul RVG,AFG & AFG2, & a KAC vert grip & the Keltec grip, XTM worked the best and felt natural). If you insist on a AFG use the Keltec one. I got the Kydex cheek cover, which is probably the best budget mod you can do IMO, helps with the cheek weld and not having your teeth ring when firing slugs. I got Magpul BUIS for it, works awesome, bolted them on and voila I was on target @ 35m w/ slugs. Still in the market for a EoTech w/ Zombie stopper reticle, a friend let me try his and it works perfect for the KSG, center reticle is for slugs, outer reticle for buck or heavy game load & the bio hazard flares are great for bird shot or light loads. <----- Best optic for a KSG IMO.
What I really want for it is the Rem-choke adaptor and One Shots' Gator duckbill shot spreader they brought in, may be ugly as sin but totally badass in every way. The other suggested mod I would tell others is the Mesa tactical rail system and shot holder, I bought one from Wanstalls but have yet to install it, haven't had the time. You may or may not have some fitting issue's but nothing a enterprising gun nut can't figure out.
For slings I found a QD Single point sling (Magpul Ms3 with QD) and a QD adaptor work best for me (it's a loop with a QD mount sewn into it, not sure where you get them found it in some old gear of mine), some don't like single points more power to you. This worked best for me.
My 2c